Molecular sieves are an important class of inorganic materials used in catalysis. Zeolites are an essential group of molecular sieves. Zeolites are crystalline aluminosilicates with a well-defined pore structure. The International Zeolite Association recognized 136 different groups of framework in January 2003. MTT is one of the structural groups of zeolites.
Four types of zeolites having MTT structure are known in the art: ZSM-23, EU-13, ISI-4 and KZ-1. ZSM-23 zeolite has one-dimensional 10-ring channels. The unit cell of ZSM-23 can be represented as: Na+n [Aln Si24-n O48]˜4H2O, n<2. The dimensions of the channel system are 0.45×0.52 nm [1].
In general, various zeolites are prepared using hydrothermal synthesis. In principal three major variables influence on the zeolite structure crystallised and said variables are the composition of the reaction mixture, the template and the time. For example, U.S. Pat. No. 3,702,886 and U.S. Pat. No. 3,926,782 describe the synthesis of MFI-structure based zeolite ZSM-5, U.S. Pat. No. 4,481,177 describes the synthesis of TON structure based zeolite ZSM-22, and U.S. Pat. No. 4,016,245 describes the synthesis of FER structure based zeolite ZSM-35.
Several publications describe different factors, which are important in zeolite synthesis. Rollmann et al. teaches about the role of small amines in zeolite synthesis [2]. The synthesis of high-silica zeolites with one-directional medium pore systems, by using nitrogen-free templates has been studied by Giordan et al. [3]. Moini et al. [4] describe the role of diquaternary cations as directing agents in zeolite synthesis. The present inventors have studied in detail the effect of synthesis time and mode of stirring on physicochemical and catalytic properties of ZSM-5 zeolite catalysts [5].
Several methods for preparation of Me-structures are known. U.S. Pat. No. 4,076,842, U.S. Pat. No. 4,104,151, GB 21 90910, GB 2 202 838, U.S. Pat. No. 4,490,342, U.S. Pat. No. 4,531,012 and U.S. Pat. No. 4,619,820 describe the synthesis of ZSM-23 from reaction mixtures containing different amines as structure directing agents or templates. International patent applications WO 96/29285 and WO 96/29286 provide methods for the production of medium pore zeolites, including ZSM-23, in the absence of any nitrogen-containing templating agent other than the small neutral amine. U.S. Pat. No. 5,332,566 teaches the formation of ZSM-23 with a templating agent having the formula C14H36N33+. U.S. Pat. No. 6,099,820 discloses a method for making MTT zeolites without an organic template. EP 0 347 273 provides a method for the manufacture of a MTT type zeolite using fluorine containing compounds in the synthesis mixture.
Isomerization is a hydrocarbon transformation reaction and it is catalyzed by acid sites. In an isomerization reaction, the molecular formula of one substance does not change but its structure changes. Isomerization can be divided into several groups after the group of molecules that are isomerized (paraffin isomerization, olefin isomerization, etc.), or alternatively it can also be divided after the reaction type (skeletal isomerization, double-bond isomerization, etc.).
The term “skeletal isomerization” stands here for a reaction wherein one n-olefin reacts to a corresponding isoolefin. This reaction is also known as olefin isomerization, hydrocarbon conversion, preparation of branched olefins, conversion of normal olefins to branched olefins and structural isomerization.
Several patents describe skeletal isomerization of olefins with zeolite catalysts. For example, in patents EP 0574 994, EP 0 523 838, U.S. Pat. No. 5,491,296, U.S. Pat. No. 5,510,560, U.S. Pat. No. 6,323,384 and U.S. Pat. No. 6,111,160 ferrierites are used in skeletal isomererization. ZSM-35 has a similar type of zeolite structure as ferrierite and its use in skeletal isomerization is discussed in U.S. Pat. No. 5,449,851 and WO 94/08920.
Other potential catalysts for skeletal isomerization of olefins are catalysts based on TON structure like ZSM-22. In U.S. Pat. No. 5,157,194, U.S. Pat. No. 5,237,121 and EP 0 5 49 294 the use of ZSM-22 based catalysts in skeletal isomerization is described.
MTT structure based catalysts, such as ZSM-23, are also used in skeletal isomerization of light olefins. In an example of U.S. Pat. No. 5,243,090, HZSM-23 is used in n-butene conversion under a pressure of 720 kPa and at temperatures between 551-554° C. The conversion of n-butene was 38% with 84% selectivity to isobutene. In U.S. Pat. No. 5,817,907, in examples 3 and 4, the uses of fresh and coked ZSM-23 catalysts in skeletal isomerization of n-butene are shown. The conversion of n-butene with the coked catalyst was 46% with 40% isobutene selectivity, and with the fresh catalyst, 52% with 20% isobutene selectivity, respectively.
Recently, a summary of the performance of different zeolite catalysts used in the skeletal isomerization has been presented [6]. It was found that ZSM-5, ZSM-22 and ZSM-23 are less adequate for butene skeletal isomerizarion than ferrierite, due to structural characteristics and a less suitable acidity (Table 1). All the studied zeolite catalysts showed a limited activity to isoolefins and a fast deactivation in the olefin skeletal isomerization reaction.
TABLE 1Microporous catalysts and their propertiesin butene skeletal isomerization [6]n-C42=i-C42=TOS-maxCatalystSi/Al ratioConversion (%)Selectivity(%)(h)Ferrierite5750901200ZSM-52755251ZSM-2226873671ZSM-225328805ZSM-2360456520SAPO-11Al/Si = 11.256652CoAPO-11Al/Co = 75399824
In the light of the state of the art it can be seen that there is an evident need for a highly active and selective zeolite catalyst, which can be used in olefin isomerization reactions.